[electrum] make the functional tests current-working-directory agnostic

Summary:
Previously the functional test could only be run from the electrum root directory. This is a bit inconvenient if we want to run them e.g. from the ${PROJECT_ROOT}/build directory.

Also use os.path.join everywhere instead of concatening strings with unix-style path separators.

Test Plan:
from the monorepo root:

pytest electrum/electrumabc/tests/regtest
cd electrum
pytest electrumabc/tests/regtest
cd ../build
pytest ../electrum/electrumabc/tests/regtest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14274